1. Technical Field
The present invention generally relates to a method for producing polymer foam by extrusion. More particularly, the present invention relates to a method for producing low-density, high melting point polymer foam by extrusion.
2. Background Information
For some time, low-density polystyrene foam (also referred to as "atactic" polystyrene foam) has been found useful in insulation, packaging, beverage cups and food containers. However, polystyrene foam generally has a service temperature limit of about 366.3.degree. K. Above the service temperature limit, items made from the foam will warp and distort. Therefore, there is a general desire for other types of low-density foam that do not suffer such shortcomings.
Polymer resins, such as, for example, poly(ethylene terephthalate) (often referred to as "PET") and syndiotactic polystyrene, exist that could be used without such shortcomings. PET is currently widely used to make many recyclable plastic items, such as soda bottles. However, attempts to produce low-density foam from polymers with "high" melting points (i.e., greater than 505.degree. K) has proven to be difficult, and the quality of such foam has been poor. Polymer foams extruded using single traditional blowing agents, such as, for example, carbon dioxide, chloro-difloro-methane and butane, have experienced foam cell collapse and/or severe corrugation. Thus, the quality of the foams produced has not been close to polystyrene. The problem of collapse is due to the high foaming temperature such materials demand, for example, PET foams at about 516.degree. K. Using conventional blowing agents at such temperatures results in a high rate of expansion, causing cell wall rupture and allowing the gas to escape. Without gas in the foam cells prior to cooling, the cells cannot support themselves. In addition, many such polymer resins are crystalline in nature, and as such, have an inferior melt strength compared to polystyrene resins. As one skilled in the art will know, melt strength refers to the ability of a material to be stretched at its melting temperature without breaking. The combination of a lower melt strength and higher vapor pressure at the foaming temperature also requires a reduction in the size of the extrusion die opening where the foam exits. Such small die openings lead to a thin gauge foam sheet experiencing severe corrugation at low densities.
Thus, a need exists for a way to make a quality low-density, high service temperature polymer foam approaching or achieving the quality of polystyrene foam.